reporter: but what she did stirred controversy, when an unkemped hannah, has a multiday affair with joshua, an older, wealthy, handsome doctor. >> you are very beautiful. >> reporter: but this morning, these mismatched mates on tv, have opened up a real-life debate online. with critics saying, he's out of her league. and calling the tryst between such different-looking and acting characters, implausible. even uncomfortable. while others say, it was charming, even evocative. why are people up in arms? >> it's unkind. bup it's human nature. >> reporter: social psychologists have long-documented our human tendency to pair off with like attractiveness. when a supermodel is paired with a less-than-supermodel, it sends shock waves. and folks assume the mismatch is made up by money, power or social status. >> it's not just physical. their need is being filled by this person. >> our thanks to juju chang. i did a story on this for "20/20" with real couples who are married. and people come up to them on the street and comment on the asymmetry of the relationship. >> causing a debate. >> it sure is.